rbl_check()
  RBL (Real-time Blackhole list) checker.
  
  This scanner can be used to scan for sender's IP address contained
  in a blacklist.
  
  Usage: rbl_check(domains)
  
  Where: domains are blacklist domain names (multiple parameters allowed).
  
  Example: rbl_check('zen.spamhaus.org')
  
  New in version 1.1.0.